A speed/position sensor is often needed for controlling an induction motor by a controller (drive), especially at close to zero speed. This is conventionally accomplished by attaching an external sensor to the motor shaft. However, the use of an external sensor is accompanied with drawbacks. Firstly, an external sensor is expensive because of the very accurate mechanical and electronic components included therein. Secondly, because an external sensor by its very nature is mounted externally, it is prone to damage and also requires installation. Thus, when accuracy demands on a sensor are low to moderate and cost and reliability demands are high, a low cost integrated sensor would be preferred.
A sensor suitable for zero and near zero speed operation should give at least two signals with a suitable phase difference (e.g. 90°) so that both speed and direction of the rotor are detectable and/or a position change is detectable at all rotor positions. These signals are further fed to the control part of the controller where they influence the voltage and/or current output signals of the controller in order to maintain commanded speed and/or rotational angle of the rotor.
Most development of prior art sensors has been aimed at modifying the motor so the d- and q-inductances differ. The rotor position can then be determined by measuring these inductances in the controller and the rotor angle determined. For example U.S. Pat. No. 6,515,395 discloses several designs of an induction motor rotor to accomplish this. However, the described solution requires specially punched laminations increasing manufacturing costs, and standard manufacturing methods are not applicable for all cases.
The German patent DE10031637 discloses a method of extracting a speed signal from a similar motor. However, the solution disclosed in this document utilizes three current sensing devices. The disclosed solution will not work at near zero speed and will not detect the direction of rotation. Further, the design of the motor is not disclosed.